The present invention is related to testing performance of wireless devices. Mobile phones, PDAs, tablet computers, and laptop computers and other wireless devices are widely used. Such devices typically communicate with a wireline computer network via an base station or cell tower, thereby allowing mobility. Various improvements in wireless technology have enabled wireless devices to be built in a smaller form factor with increased throughput and computing power. Testing is performed to mitigate the risk that devices will fail to perform as planned because the costs associated with developing, purchasing, selling and deploying a new wireless technology or products are often quite high. However, wireless device performance is notoriously difficult to predict because it can be affected by many factors. The different conditions to which a wireless device may be subjected in actual use is so great in number that it is difficult and time-consuming to create all of those conditions in a test environment.
It is known to perform open-air testing by manually moving a wireless device under test (DUT) through an open air test environment such as drive testing to predict performance. However, this technique is too labor intensive and imprecise to simulate a wide variety of traffic conditions, distances between devices and rates of motion in a practical manner. Further, such a manual, open-air test can be rendered invalid by transient interference from a microwave, RADAR or other RF source.
It is also known to perform over-the-air (OTA) testing in an anechoic chamber. Channel emulators can be used to create conditions such as delay, Doppler, and correlation. A number of antennas mounted within the chamber are used to transmit signals from the channel emulator to the DUT. This has the advantage of being a more controlled environment than an open air test. However, anechoic chambers are relatively large and costly. The techniques required to produce the channel conditions may also require a large number of channel emulator electronics and antennas. Additionally, calibration of such a system and performing isotropic measurements in such a system are typically very time consuming.
It is also known to perform OTA testing in a reverberation chamber. The reverberation chamber has walls that reflect electromagnetic waves so a signal transmitted within the chamber tends to reverberate, creating standing waves with randomly located peaks and nulls. Moveable mechanical devices called “stirrers” are used to change the location of the peaks and nulls. However, mechanical systems are not well suited to providing Doppler conditions similar to those experienced by an actual DUT in fast motion, such as in an automobile or train, due to the speed limits of the stirrers. Furthermore, the average chamber impulse response is a simple decaying exponential, which is very different from actual channel conditions in which the average channel impulse response is produced by a number of discrete, physically separated reflectors. Consequently, reverberation chambers are generally limited to reproducing conditions of low Doppler frequencies and simple decaying exponential power delay profiles or for testing that does not require realistic channel conditions.
It is also known to perform “conducted testing” to simulate a wireless environment. Conducted testing can be performed by bypassing the DUT antenna with direct wired connections to the DUT. The DUT is typically enclosed in an EMI-shielded container, and a channel emulator is used to create conditions such as delay, Doppler, and correlation. This technique has the advantage of being less costly than an anechoic chamber and having the capability for creating a greater range of conditions than a reverberation chamber. However, conducted testing also has some drawbacks. For example, it may be necessary to disassemble the DUT in order to bypass the antenna. Furthermore, as will be explained in greater detail below, bypassing the antenna may compromise the value of the test results as the device is not tested in the same form it will be sold or used.
Recent advances in wireless technology present even more difficult challenges to designers of test systems. One example is multiple input, multiple output (MIMO) systems. MIMO systems increase throughput using a combination of antenna design, radio design and baseband signal processing design. Although each subsystem separately affects performance and reliability, coordinated operation of the subsystems is generally required to achieve best results. Historically, conducted testing is typically performed to measure the performance of the radio and baseband processing subsystems, and OTA testing is performed to measure the performance of the antenna subsystem. OTA testing to determine the performance of the antenna subsystem may include evaluation of parameters such as total isotropic sensitivity (TIS) and total radiated power (TRP). However, coordinated operation of all the subsystems, in conjunction with the expected conditions the device will be operated in (RF fading channel conditions), and measuring performance based on a figure of merit that the end user directly experiences, such as data throughput, is very desirable for data devices and MIMO devices that are designed to offer enhanced throughput.